Many industrial processes require fluid application to a material. For instance, applying a surface coating (e.g., paint) to an object requires fluid (such as a liquid or powder slurry) to be sprayed or otherwise delivered to the material. Typically, this fluid application will occur in a contained environment that enables control of the fluid. For example, paint is often applied in a paint spray booth that allows control of the atmospheric conditions and containment of the paint. Inherent in the paint spraying process is overspray, that is, paint that does not adhere to the object being painted and floats in the air as a mist. In order to provide a safe working environment and a high quality finished product, paint spray booths require a substantially continuous supply of clean, fresh air, which also assists with discharge of the overspray from the booth.
Various configurations of spray booths have been developed for different fluid application processes and discharge of the overspray. These booths are often classified by the direction of the airflow in the spraying area. For instance, cross-draft booths include an airflow which moves parallel to the floor from behind the operator toward a dry filter or a water curtain. Downdraft booths have an airflow which moves from the ceiling vertically downward to an exhaust system below the floor. Semi-downdraft booths include an airflow which moves in a diagonal direction in the booth towards an exhaust.
Since the overspray contains paint particles, it is important to filter or otherwise clean this air before discharging it back into the environment. Several methods have been developed for separating the paint mist from the air exhaust stream. For instance, a dry method results in air entrained with paint being forced through a dry filter or screen which absorbs or otherwise captures the paint particles. A wet method requires the air entrained with paint to contact and be mixed with another fluid, such as water, so that the paint particles are captured by the fluid.
Due to the large amount of paint used by most industrial paint facilities, such as automotive plants, the wet method in a downdraft booth is the preferred configuration. These booths generally have a wet scrubber that captures the fluid overspray and assists with separating the paint particles from the air.
Over time, various configurations of wet scrubbers have been developed in attempts to increase the efficiency of the particle separation and minimize operating costs for the paint facility. For instance, a Venturi type of scrubber utilizes a restriction or Venturi to accelerate the paint laden air, generate turbulence, and break-up the supply water (or the capturing fluid) running along its walls into small drops that capture or trap within the droplets the paint particles in the exhaust air. Another type consists of an elongated tube with a nozzle positioned at the tube exit, whereby water flows downward along the walls of the tube and into a pool of liquid contained within a capturing chamber, in which turbulence is generated and the paint particles in the air are captured or trapped within the water. Although these designs capture the overspray and separate some of the paint particles, they use a large amount of energy and/or allow a sub-optimal amount of paint particles to penetrate the system and be exhausted to the outside air. More modern scrubber designs utilize vortex chambers to capture and separate paint particles and volutes to decelerate the air flow and recover pressure. Although these designs capture a greater amount of paint particles than the venturi or tube types, the volutes permit back flow of the exhaust, which perturbs the vortex chamber flow, thereby decreasing both capturing efficiency and pressure recovery, that is, increasing effective pressure drop. This back flow in the volutes generates recirculation zones that reduce the effective flow area precluding appropriate deceleration of the flow and, hence, reducing pressure recovery. In addition, the energy required to sustain the recirculation zones is taken away from the flow, therefore, reducing the recoverable pressure energy.
Accordingly, the particle separation arts have need for a more efficient scrubber; that is, a scrubber that captures or traps a desired amount of particles (increasing capturing efficiency), while minimizing pressure drop.